1. Field of the Invention
This invention relates to a ball joint having a housing integrally formed of a synthetic resin and to a method of manufacturing this ball joint.
2. Description of the Prior Art
Ordinarily, a ball joint is constitutes by a ball stud and a housing. A spherical head portion of the ball stud housed in a spherical inner chamber formed in the housing, while a shaft portion of the ball stud is movably projects outward through an opening of the housing. Ball joints of this construction are widely used for vehicle suspensions or adapted for other kinds of use.
Ball joints of this construction entail a problem in that the inside diameter of the opening of the housing must be smaller than the outside diameter of the spherical head portion of the ball stud, and it is possible that the ball stud cannot be inserted or, conversely, it will come off the housing, unless these members are manufactured with the desired control of the relationship between their sizes.
Conventional ball joints are disadvantageous in terms of weight and the problem of rust because they are formed of metals. For this reason, the development of resin-made housings has recently been promoted; ball joints in which the housing constitutes by an assembly of separate synthetic resin parts or is integrally formed of a synthetic resin have already been developed.
For example, Japanese Utility Model Laid-Open No. 62-41,925 discloses a ball joint having a construction such that a spherical head portion of a ball stud is inserted in an inner chamber of a housing integrally formed of a synthetic resin, and an opening portion of the inner chamber and a dust cover are fastened together by a fastening ring. In this publication, it is stated that, for setting the relationship between the inside diameter d of the opening and the inside diameter D of the inner chamber, the most suitable value of (D-d)/D is 0.05 to 0.1.
In conventional ball joints, a dust cover is mounted between the shaft portion of the ball stud and the housing in order to prevent water, dust and the like from entering the gap between the slide surfaces of the housing and the spherical head portion of the ball stud through the opening, and to thereby prevent the ball stud from rusting as well as to maintain the smoothness of the flexing motion.
Conventional ball joints formed of synthetic resins, however, entail various problems relating to coming-off of the ball stud, the rigidity of the housing, the durability and the manufacturing cost of the ball joint, and the sealing properties and the attachment of the dust cover, and so on.
In the case of the ball joint disclosed in the above publication, as explained in the specification thereof, the mold releasability of the molding is so reduced as to cause a risk of deformation or breakage of the housing if the value of (D-d)/D is 0.1 is larger, or there is a risk of the ball stud coming off the housing if this value is 0.05 or smaller.
A type of ball joint, such as the one disclosed in Japanese Utility Model Laid-Open No. 62-75,217, is known in which the housing consisting of two separate parts is attached to the spherical head of the ball stud with an elastic ring. This ball joint also entails the problem of deterioration of the durability and the problem of an increase in the number of parts.
In regard to the dust cover, the use of a metallic ring or the like for fastening the dust cover and the housing increases the number of parts, which is also disadvantageous in terms of cost.
Fastening with a metallic ring is advantageous in terms of prevention of coming-off of the dust cover but entails a drawback because the use of the metallic ring makes the attachment operation difficult.
The following problem is encountered with respect to the rigidity of the housing. In a ball joint assembly consisting of two conventional ball joints with a synthetic resin housing, it is difficult to align the centers of the spherical head portions of the ball studs and the cross-sectional centroid of a housing rod portion which connects the two inner chambers, because dust covers are fastened to the outer circumferences of the opening portions of the housing. This will be explained below with respect to a ball joint assembly shown in FIG. 20. It is necessary to form annular recesses in the opening outer circumference portions of the housing 101, with which recesses the lower end flanges of dust covers 109 are engaged. The cross-sectional centroid 129 of the boundaries between the rod portion 132 and the portions in which ball studs 102 are housed (hereinafter referred to as socket portions 131 of the housing) is necessarily set below an axial line 130 connecting the two spherical portions, as viewed in FIG. 20. Consequently, if the shaft portions of the two ball studs 102 are pulled in the directions of the arrows as shown in FIG. 20, this tensile force is applied along the axial line 130 connecting the spherical portions and causes bending of the housing 101 alone, as indicated by the double-dot-dash line, since the cross-sectional centroid 129 of the rod portion 132 does not coincide with the axial line 130. In this event, therefore, there is a risk of each ball stud 102 coming off the inner chamber.